Fix type checker errors in privacy/wrappers.py

[aviralgarg05]

Mypy reports several type incompatibility issues in src/agentunit/privacy/wrappers.py:

1. Incompatible Return Type: get_private_dataset returns None (via _masked_dataset) but is typed to return list[DatasetCase].
2. Assignment Error: Metadata assignment in _mask_case has incompatible types.
3. Argument Error: DatasetCase constructor call has incompatible metadata type.

These should be resolved to ensure the codebase passes strict type checking.